Optical interference, that modifies the transmitted and reflected intensities of light, occurs with the superposition of two or more beams of light. The principle of superposition states that the resultant amplitude is the sum of the amplitudes of the individual beams. The brilliant colors, for example, which may be seen when light is reflected from a soap bubble or from a thin layer of oil floating on water are produced by interference effects between two trains of light waves. The light waves are reflected at opposite surfaces of the thin film of soap solution or oil.
More importantly, a practical application for interference effects in thin films involves the production of coated optical surfaces. When a film of a transparent substance is deposited on transparent substrate such as glass, for example, with a refractive index which is properly specified relative to the refractive index of the glass and with a thickness which is one quarter of a particular wavelength of light in the film, the reflection of that wavelength of light from the glass surface can be almost completely suppressed. The light which would otherwise be reflected is not absorbed by a non-reflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a concomitant increase in the intensity of the light which is transmitted.
Considerable improvements have been achieved in the anti-reflective performance of such films by using a composite film having two or more superimposed layers. Two different materials may be used in fabricating such a composite film, one with a relatively high index of refraction and the other with a relatively low index of refraction. The two materials are alternately deposited to predetermined thicknesses to obtain desired optical characteristics for the film. In theory, it is possible with this approach to design multi-layer interference coatings for a great variety of transmission and reflection spectra. This has led to the development of many new optical devices making use of complex spectral filter structures. Anti-reflection coatings, laser dielectric mirrors, television camera edge filters, optical bandpass filters, and band rejection filters are some of the examples of useful devices employing thin film interference coatings.
One particular type of interference coating is the optical bandpass filter, which is designed to allow wavelengths within a predetermined range (i.e., the desired pass-band) to be transmitted, while a range of wavelengths on either side of the pass band are highly reflected. Ideally a bandpass filter should have a square spectral response. In other words, the transition from the rejection regions to the passband should be as rapid as possible, or expressed differently, the slope or transition region should be as steep as possible, while obtaining a pass band region that is uniform and has little or no ripple.
The simplest bandpass filter, shown in prior art FIG. 1, consists of two partial-reflectors or semi-mirrors separated by a half wave layer of transparent dielectric material (e.g., glass or air).
Turning now to FIG. 2, for all-dielectric filters, the partial-reflector shown consists of alternating layers of high and low index materials. Each layer is deposited as a quarter-wave (QW) thickness at the wavelength of the desired filter. Each partial-reflector, which may be comprised of only a single layer, is called a quarter-wave stack (QWS). The bandwidth of the filter is a function of the reflectance of quarter-wave stacks in the structure. The centre wavelength of the pass-band is determined by the thickness of the spacer dielectric material.
Referring now to FIG. 3, a filter cavity, the basic building block for all-dielectric interference filters, is shown. The cavity is comprised of two reflectors made from quarter-wave stacks separated by a half wave (or multiple halfwave) dielectric layer. Notably, the cavity essentially functions as an etalon or a Fabry-Perot filter.
Cavities are deposited on top of other cavities, with a quarter-wave layer of low index material between, to sharpen the slopes of the transmission response. This produces a multi-cavity filter as shown in FIG. 4.
Typically, the cavities are deposited on a substrate that is transparent over the wavelength of interest and, may be made from a wide variety of materials including (but not limited to), glass, quartz, clear plastic, silicon, and germanium. Usually, the dielectric materials used for the quarter and half-wave layers have indices of refraction in the range 1.3 to beyond 4.0. For example, some suitable materials are: Magnesium Fluoride (1.38), Thorium Fluoride (1.47), Cryolite (1.35), Silicon Dioxide (1.46), Aluminum Oxide (1.63), Hafnium Oxide (1.85), Tantalum Pentoxide (2.05), Niobium Oxide (2.19), Zinc Sulphide (2.27), Titanium Oxide (2.37), Silicon (3.5), Germanium (4.0), and Lead Telluride (5.0). Of course, other dielectric materials would serve as well.
Design of the filter is easily accomplished with the aid of commercially available computer programs with optimization routines (i.e. TFCalc.™ by Software Spectra Inc.). In particular, the design is entered into the program and the spectral response is calculated. When the design with the appropriate cavity size is selected to match the required nominal bandwidth, optimization of the filter transmission is performed for the matching layers. The designer selects from a choice of materials to use in a quarter wave match or may choose to use the same low index material with thickness adjustments to accomplish the matching.
There have been various attempts to improve the spectral performance of the multi-cavity filter shown in FIG. 4. In particular, there has been significant emphasis on improving transmission, producing a square band shape, and reducing ripple. For example, U.S. Pat. Nos. 5,719,989, 5,926,317, 5,999,322, 6,011,652 and 6,018,421 to Cushing, incorporated herein by reference, disclose varying the number of cavities, varying the number of layers in the outer cavity stacks, adding half wave to inter cavity stacks, and/or adding multiple half wave to the outer cavity stacks. In each of these references the cavities and/or modified cavities are symmetrically arranged about the centre of the filter.
What has not been addressed in the field of multi-cavity filters is chromatic dispersion. U.S. Pat. Nos. 6,081,379, 6,256,434, 6,081,379 and 6,154,318, incorporated herein by reference, discuss the problems associated with chromatic dispersion and teach optical devices having reducing dispersion. However, none of these teachings are applicable to multi-cavity filters. U.S. Pat. No. 6,278,817 to Dong teaches a Bragg grating filter designed such that reflected dispersion at one end of the grating element is decreased at the expense of increased dispersion at the other end of the grating. This is accomplished by providing the periodic variations in refractive index with a spatially asymmetric index of modulation. However, since multi-cavity filters operate differently and have a different structure from fibre Bragg gratings, these teachings do not provide a solution to lowering the dispersion in multi-cavity filters.
It is an object of the instant invention to provide a low reflected dispersion multi-cavity filter.